034S Idle Issues

[drf256]

Here’s one I found interesting.

Freshly built 034S with new Zama C3A off eBay.

Saw would hold high tune all day, idle all over the place.

Obviously a vac leak, it’s the classic sign right?

Saw has new oem gasket set/seals and 518 as jug base sealant.

 

[drf256]

Didn’t vac test after the build because all went in perfect. Had to pull apart to test.

 

[drf256]

WTF?

Time to look at the new carb.

 

[drf256]

The point of this thread is to never overlook the basics because you’ll chase your tail.

Changing the $2 spark plug was the next move.

The needle and seat were bad on the new Zama. Fuel would fill the wet side and dribble into carb bore causing all sorts of odd stuff at idle. A rebuild of an oem Jap Zama solved the problem.

Hope this helps someone.

 

[redlight066]

Truer words....... Glad you got to the bottom of it. For those out there that don’t have a pressure/vac tester, this proves it’s the single most important tool on the bench. Trust but verify

 

[trooney]

I like Cream much betterer.....

 

[angelo c]

I like Cream much betterer.....

You're too young to fully appreciate quality "Rock and Roll...."

 

[BuckthornBonnie]

Nice thread. I’ve had bad carbs before as well. One oem 460 carb had a bad seat and would tune fine, but would eventually flood the saw (especially when sitting).
Would that saw die at idle or just go all over for rpms?

 

[Stump Shot]

Excellent thread!
While pressure testing rebuilt carburetors is standard procedure for me, a new one isn't necessarily on the radar for potential problems before use.
I keep a glass jar of water on the bench for when there is a leak the carb can be dunked in with hose attached to see just where the bubbles are coming from to make short work out of it.

Here's a picture of one leaking in the impulse circuit, this was from a warped main body. After being re-trued it was fine. Without pressure testing, something like this is near impossible to diagnose.

 

[drf256]

Nice thread. I’ve had bad carbs before as well. One oem 460 carb had a bad seat and would tune fine, but would eventually flood the saw (especially when sitting).
Would that saw die at idle or just go all over for rpms?

All over untunable rpm, then also stall if sitting too long or position rotated.

I woulda bet good money it was a vac leak, but it was not.

 

[Mastermind]

One thing I'd like to add.

I do a lot of carb mods....and so I pressure test a bunch of carbs. I thing I never worry about is pop off pressure. Now I know a bunch of posts talk about testing this, but in all honesty, I've never seen a reason to care. I'm sure there are situations were lowering, or raising POP matters, but on the saws I build, I've yet to see it.

That said....

It's a good idea to get in a habit of pressure testing carbs. On any used saw I build, I always make sure that the carb will hold 15psi, and that it will reseal after the metering lever is tripped. Once in awhile you will find one that regardless of what you do, the needle and seat just won't hold pressure.

Sometimes it's possible to polish the seat with a Q-Tip dipped in polishing compound. Sometimes, they are just out of round and that's nearly impossible to remedy.

 

[qurotro]

C3M from OE 365 run just GOOD. But sat over night and cold start next morning would be ridiculously rich in the first few rev. The needle is leaking mix to the engine by a small amount. Change needle fix it.

 

[Canadian farm boy]

Say if I have a carb that pops at 15psi at at what pressure should it reseal?

I currently have one that pops at 15psi and doesn’t reseal completely until approx 5psi.

 

[Canadian farm boy]

I should add that I’m going to be putting a kit in this carb regardless just for my own peace of mind.

I’m more just curious if this is what you guys would call “normal”

 

[MG porting]

The point of this thread is to never overlook the basics because you’ll chase your tail.

Changing the $2 spark plug was the next move.

The needle and seat were bad on the new Zama. Fuel would fill the wet side and dribble into carb bore causing all sorts of odd stuff at idle. A rebuild of an oem Jap Zama solved the problem.

Hope this helps someone.

That's one of the most over looked problem with the Zama carbs never did like them all that much.

 

[redlight066]

I recently had the pleasure of working behind another “mechanic”. Initial inspection looked good, gaskets and diaphragm we’re all but new. Passed a pressure test. If I didn’t work on this stuff every day I never would have noticed the wrong spring had been installed in a Walbro HD. Spring rate was so high the needle would barely crack open at partial throttle start up and seal up tight at “idle “. Installed the correct spring and it ran fine.

 

[dmb2613]

Say if I have a carb that pops at 15psi at at what pressure should it reseal?

I currently have one that pops at 15psi and doesn’t reseal completely until approx 5psi.

It should reseal soon as it hits the seat

 

[Simondo]

If you have to diagnose carbs on many 2 stroke motors then for your effectiveness and might i add ...Sanity !! then one of these is one of your best workshop friends IMO....


As Randy mentions....15 psi on the metering pop off is as good a number as any ...but ... its the reseat pop off that I pay the most attention to. That needle "MUST" reseat again holding pressure after the diaphragm trips the needle off the seat and then pulls back. Always do the test with a wet needle pocket as well.

 

[Mastermind]

Say if I have a carb that pops at 15psi at at what pressure should it reseal?

I currently have one that pops at 15psi and doesn’t reseal completely until approx 5psi.

I have some little bitty brass washers I use under the spring if a little more pressure is needed.

 

[Wonkydonkey]

Another incredible thread.. very simple but,,,, very interesting. Easy stuff to over look and end up chasing your tail...all day and the next day as well

Cheers